How does sewage kill fish?
Reveal answer
Organic matter → bacteria multiply → respire → O₂ falls → fish die.
Key words: bacteria · oxygen
5(d) HUMAN INFLUENCES ON THE ENVIRONMENT · RECALL
5.16 Water pollution by sewage
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Indicator of clean water?
Reveal answer
Mayfly / stonefly larvae.
Key words: mayfly
Indicator of polluted water?
Reveal answer
Sludge worm / rat-tailed maggot.
Key words: sludge worm
Health risk from sewage?
Reveal answer
Pathogens e.g. cholera.
Key words: pathogens
Why does oxygen recover downstream?
Reveal answer
Sewage used up → fewer bacteria → O₂ dissolves back.
Key words: fewer bacteria
What is an indicator species?
Reveal answer
A species whose presence shows pollution level.
Key words: pollution
Method to sample stream invertebrates?
Reveal answer
Kick sampling with a net.
Key words: kick
Why does warm water worsen sewage pollution?
Reveal answer
Less dissolved O₂ + faster bacterial respiration.
Key words: less
What does high BOD mean?
Reveal answer
Lots of organic matter → oxygen will be used up.
Key words: oxygen
What is meant by the term sewage?
Reveal answer
Waste water containing human and animal waste and other material.
Key words: sewage
What is meant by the term indicator species?
Reveal answer
An organism whose presence or absence shows the level of pollution.
Key words: indicator species
Effects downstream of a sewage outflow: for Upstream (clean), give the dissolved oxygen and the typical organisms.
Reveal answer
Dissolved oxygen: High. Typical organisms: Mayfly and stonefly larvae, fish.
Key words: Upstream (clean)
Effects downstream of a sewage outflow: for At the outflow, give the dissolved oxygen and the typical organisms.
Reveal answer
Dissolved oxygen: Falling as bacteria multiply. Typical organisms: Bacteria increase; fish leave or die.
Key words: At the outflow
Effects downstream of a sewage outflow: for Just downstream, give the dissolved oxygen and the typical organisms.
Reveal answer
Dissolved oxygen: Lowest. Typical organisms: Sludge worms, rat-tailed maggots.
Key words: Just downstream
Effects downstream of a sewage outflow: for Far downstream, give the dissolved oxygen and the typical organisms.
Reveal answer
Dissolved oxygen: Recovers as organic matter is used up. Typical organisms: Sensitive species gradually return.
Key words: Far downstream
True or false, and why? “Sewage kills fish because it is poisonous.”
Reveal answer
False. The main cause is oxygen depletion by decomposer bacteria respiring.
True or false, and why? “Bacteria in sewage use up oxygen by photosynthesis.”
Reveal answer
False. Bacteria use oxygen in aerobic respiration.
True or false, and why? “If water looks clear, it is free of pathogens.”
Reveal answer
False. Pathogens are microscopic, so clear water can still be contaminated.